1. Field of the Invention
The present invention relates to a plasma etching apparatus for processing an object to be processed such as a semiconductor wafer, and a plasma etching method using the plasma etching apparatus.
2. Description of the Related Art
In a semiconductor chip manufacturing process, a plasma etching apparatus using reactive plasma to process an object to be processed such as a semiconductor wafer is conventionally used.
Here, with reference to a cross-sectional view of an object to be processed shown in FIG. 13, etching for forming a poly-silicon (Poly-Si) gate electrode of an MOS (Metal Oxide Semiconductor) transistor (hereinafter referred to as “gate etching”) will be explained as one example of a plasma etching process. As shown in FIG. 13(a), an object to be processed 1 prior to etching is formed of a silicon dioxide (SiO2) film 3, poly-silicon film 4 and photoresist mask 5 deposited on the surface of a silicon (Si) substrate 2 in the named order. This photoresist mask 5 is formed via a photolithography process by applying a photoresist, projecting the same pattern onto one or a plurality of chips for exposure to light using a mask called a “reticule” through a reduced projection photolithography apparatus, and developing the same. The dimension of this photoresist mask 5, that is, a photoresist mask width 7, greatly affects the width of a gate electrode which will be described later, and is therefore subject to strict control.
Gate etching is a process for removing the poly-silicon film 4 in an area not covered with the photoresist mask 5 by exposing the object to be processed 1 to reactive plasma, and by this process, a gate electrode 6 is formed as shown in FIG. 13(b). Since a gate width 8 at the bottom of the gate electrode 6 greatly affects the performance of the electronic device, it is subject to strict control as most important CD (critical dimension). For this reason, a target completion dimension is preset for the gate width 8.
Furthermore, a value obtained by subtracting the gate width 8 after etching from the photoresist mask width 7 before etching is called a “CD shift”, and constitutes an important index for expressing the quality of gate etching.
A conventional example of a plasma etching apparatus which carries out the above described gate etching will be explained with reference to FIG. 14. A processing chamber cover 12 is placed on a quasi-cylindrical processing chamber side wall 11, and a processing chamber 13 defined by the above parts is provided with a substrate holder 14.
A processing gas 21 is introduced into the processing chamber 13 through an inlet 22 provided in the central part of the processing chamber cover 12 to generate plasma 25. Plasma etching is performed by exposing the object to be processed 1 to this plasma 25. The processing gas 21 and a volatile substance generated by the reaction in the plasma etching processing are exhausted from the outlet 30. A vacuum pump (not shown here) is connected to the tip of the outlet 30 and the pressure in the processing chamber 13 is thereby reduced to approximately 1 Pa (Pascal).
Gate etching is performed using the plasma etching apparatus as described above, but with the recent increase in the diameter of the object to be processed 1, it is becoming more difficult to secure the in-plane uniformity of etching rates or the in-plane uniformity of the gate width 8 over a wide area of the object to be processed 1. Likewise, along with the recent miniaturization of semiconductor devices, demands are increasing for more severe dimensional control of the gate width 8.
Next, adhesion and deposition of reaction products onto the side of a gate electrode, which is one of influences on the dimension of the gate width 8, will be explained. A plurality of gases such as chlorine (Cl2), hydrogen bromide (HBr) and oxygen (O2) are conventionally used for processing of gate etching. During etching, these gases are in a plasma state to perform etching on the poly-silicon film 4, but during the process, chlorine, hydrogen bromide and oxygen contained in the processing gas 21 are dissociated, and the thus-produced ions and radicals of Cl (chlorine), H (hydrogen), Br (bromine) and O (oxygen) react with silicon deriving from the poly-silicon film 4, producing reaction products. Of these reaction products, volatile ones are exhausted from the outlet 30, but non-volatile products are adhered or deposited onto the inner side (vacuum side) of the processing chamber side wall 11, the processing chamber cover 12 and the sides of the poly-silicon film 4 and photoresist mask 5. When the reaction products are deposited on the sides of the poly-silicon film 4 and photoresist mask 5, they serve as a mask for etching, which often increases the gate width 8.
Especially when a compound SiBrx (x=1, 2, 3) of silicon and bromine or compound SiClx (x=1, 2, 3) of silicon and chlorine reacts with oxygen (O), SixBryOz (x, y, z: natural number) or SixClyOz (x, y, z: natural number) which are non-volatile and have high deposition characteristic is produced, and adhesion or deposition of these products to the poly-silicon film 4 and photoresist film 5 may cause an increase of the gate width 8.
The increase of the gate width 8 may occur nonuniformly within the plane of the object to be processed 1. That is, nonuniform CD shifts may occur within the plane of the object to be processed 1. For example, in an area with a high etching rate, the concentration of reaction products including silicon deriving from the poly-silicon film 4 becomes higher than in areas with a low etching rate, which may cause in-plane nonuniformity of CD shifts.
Furthermore, in the central part of the object to be processed 1, all surrounding areas are subject to etching, whereas outside the outermost region of the wafer, there is no area subject to etching. For this reason, even if an etching rate is uniform within the plane of the object to be processed 1, the concentration of reaction products including silicon deriving from the poly-silicon film 4 is high in the center portion and low in the outer regions. This may also cause in-plane nonuniformity of CD shifts.
Furthermore, as described above, reaction products are deposited on the processing chamber side wall 11 or inner side (vacuum side) of the processing chamber cover 12 through plasma etching processing, but during plasma etching, radicals and ions of chlorine, hydrogen, bromine, oxygen and these compounds may be dissociated from these depositions and discharged into the plasma 25. In this case, the concentration of the radicals and ions discharged from the reaction products is likely to increase in the outer regions of the object to be processed 1. This is because the processing chamber cover 12 is placed parallel to the object to be processed 1 as shown in FIG. 14, and radicals and ions discharged from the deposited reaction products are likely to disperse over the whole object to be processed 1, while the processing chamber side wall 11 is located to surround the outer regions of the object to be processed 1 and radicals and ions discharged from the reaction products deposited thereto are likely to cause an increase of concentration in the outer regions of the object to be processed 1. The radicals and ions discharged from the reaction products as described above may cause deterioration of in-plane uniformity of CD shifts on the surface of the object to be processed 1.
As described above, nonuniformity of concentration of reaction products is caused on locations within the plane of the object to be processed 1, but this nonuniformity varies from moment to moment according to the condition in the processing chamber 13. That is, even if the total amount and composition of the processing gas 21 or process input conditions such as the pressure in the processing chamber 13 are the same when plasma etching is performed, CD shifts fluctuate. This is because the adhesion condition of reaction products deposited on the processing chamber cover 12 and processing chamber side wall 11 varies from moment to moment as the plasma etching processing advances as described above.
In addition to the advance of the above described plasma etching processing, the condition in the processing chamber also changes through a process called “cleaning.” Every time the aforementioned plasma etching process is carried out, the amount of reaction products deposited on the inner side (vacuum side) of the processing chamber side wall 11 and the processing chamber cover 12 increases. When these depositions fall off and attach to the surface of the object to be processed 1, the yield of volume production of semiconductor devices is deteriorated. To prevent this, plasma cleaning using reactive plasma is carried out periodically to remove the aforementioned depositions. Furthermore, depositions which cannot be removed by plasma cleaning are removed by operations called “wet cleaning” or “manual cleaning” which are manually performed by the operator with the processing chamber 13 left open to the atmosphere. These two types of cleaning processes reduce the amount of depositions stuck to the processing chamber cover 12 and processing chamber side wall 11. As shown above, since the condition in the processing chamber 13 varies from moment to moment, distributions of radicals and ions on the surface of the object to be processed 1 also change accordingly.
In the plasma etching apparatus of the conventional example (prior art) shown in FIG. 14, the processing gas is only introduced from the inlet 22 provided above the central part of the object to be processed 1, and therefore the concentration of radicals of gas components contained in the processing gas or ions resulting from dissociation is often high in the central part and low in the outer regions of the object to be processed 1.
One art intended to improve in-plane uniformity of ions and radicals in plasma is an art of introducing a processing gas from a plurality of parts of the processing chamber. This art relates to a reactive ions etching apparatus provided with a flow rate controller capable of introducing the processing gas into the processing chamber through a plurality of inlets and controlling the flow rate of the processing gas for each inlet independently (e.g., see Patent Document 1). This art is capable of changing the in-plane uniformity of the etching rate, but since the processing gas introduced from the respective inlets has the same composition, it cannot sufficiently adjust the in-plane uniformity of ions and radicals.
There is another art of introducing a reaction product gas into the processing chamber for the purpose of improving the concentration distribution of reaction products on the surface of the object to be processed 1. This art relates to a method of dry etching which provides two gas inlets, introduces a reactive gas from one inlet and introduces a reaction product gas generated by an etching reaction from the other inlet as a reaction inhibition gas for the purpose of equalizing the etching rate on an object to be processed (e.g., see Patent Document 2). The use of this method can adjust the in-plane uniformity of ions and radicals and improve in-plane uniformity of the etching rate.
However, since the position of introducing the reaction product gas as the reaction inhibition gas is limited to one inlet, this structure has constraints on the improvement of in-plane uniformity of the etching rate. For example, when the etching rate in the central part of the object to be processed is greater than the etching rate in the outer regions, it is possible to improve the in-plane uniformity of the etching rate by introducing a reaction product gas into the central part as the reaction inhibition gas. However, on the contrary, when the etching rate in the outer regions of the object to be processed is greater than the etching rate in the central part, this structure requires gas pipes to be replaced, so it is unable to respond to the demand quickly. It also has the disadvantage that a supply source of the reaction product gas and piping system need to be added in addition to the gas used for normal etching to the apparatus.
In view of the above described problems, it is an object of the present invention to provide a plasma etching apparatus and plasma etching method capable of carrying out processing with excellent in-plane uniformity on an object to be processed having a large diameter.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 62-290885
[Patent Document 2]
Japanese Patent Application Laid-Open No. 5-190506
[Patent Document 3]
Specification of U.S. Pat. No. 6,418,954